The patent applications, patents, and paper cited above disclose in more detail a typical gas-fired cooking apparatus and components thereof in which heat exchangers according to the present invention are employed to advantage.
Such a gas-fired double-sided griddle for commercial cooking typically includes a lower platen with a horizontal top cooking surface and an upper platen with a bottom cooking surface. A positioning mechanism moves the upper platen of each griddle into a horizontal position in contact with the food, permits the upper platen to press downward on the food until it reaches a predetermined level above the lower platen, stops the downward movement at that level, and lifts the upper platen away when the food is cooked. In order to achieve uniform doneness, all food on the griddle should be subject to a constant temperature cooking surface. The heat exchangers described here achieve temperature control by the intermittent flow of a heated fluid, typically a heat transfer oil.
A supply of oil is heated to desired temperatures. The heated oil (maintained at substantially constant pressure by a weight controlled valve) is circulated through a first heat exchanger, which contacts, and transfers heat to, the lower platen, and through a second heat exchanger, which contacts, and transfers heat to, the upper platen.
Each heat exchanger according to this invention typically comprises three contiguous flat metal layers. The exterior layers are flat sheets that provide covers for the inner layer. The inner layer has a passage for the oil to flow through, to heat an outer layer and maintain a substantially uniform temperature over its outer surface. The present invention comprises a particularly advantageous configuration for the oil flow passage. The shape and dimensions for the passage may be determined in part by finite element or finite difference thermal conduction computer programs. The passage may be cut out by a laser so as to minimize stresses in the inner plate and differences in thermal expansion among the layers.
Some prior art has addressed the issue of uniform heat exchanger temperature, for example, Friedman et al. U.S. Pat. No. 4,478,227, issued Oct. 23, 1984, for Heat Exchanger Having Uniform Surface Temperature and Improved Structural Strength. The present device is less expensive and appears to be superior to those in the prior art because of better local uniformity and better control of the edge heat losses.